The overall objective of the proposed research is to study both the normal and visually deprived development of the cat lateral geniculate nucleus. Specifically, the relative effects of deprivation on the various sub-classes of geniculate cells; i.e., in terms of both size and morphology, are being studied. Briefly, the following procedures are used. Injections of H3-thymidine are made in kitten fetuses at various times during their prenatal development. The available thymidine is taken up by the cells that are dividing in each of the kitten fetuses, thus 'tagging' sub-populations of geniculate cells with similar birthdates. Once all separately labeled sub-populations of geniculate cells have been radioactively 'tagged', it will be possible to study the normal and visually deprived sequence development for each of these sub-populations. By also making cortical injection of horseradish peroxidase (HRP) in the same animals it will be possible to study the general morphology of cells in each of the geniculate sub-populations. These studies will afford us with the information necessary to determine whether or not all geniculate cells are affected equally by deprivation. This information is especially important in understanding the mechanisms that underlie normal and visually deprived development since the different sub-classes of geniculate cells are likely to subserve quite different visual functions.